The microcapsule is in broad use in the fields of thermo-color materials, perfumery, agrochemicals, pressure-sensitive copying and recording materials, etc. The known microencapsulation technology includes in-situ polymerization, interfacial polymerization, coacervation, in-liquid drying, spray drying and other methods. Among these methods, in-situ polymerization is advantageous that it does not require much rigorous production control and the necessary encapsulation can be easily achieved in a comparatively short time.
Particularly the in-situ polymerization process using an amino resin as the wall material is conducive to microcapsules of improved capsule strength. Therefore, this process is predominating over other processes in the manufacture of microcapsules for the fields of use requiring a high core-retaining property. Many development and improvement efforts have been made in connection with water-soluble polymers (system modifiers) for use as electrolytes in this method and, by way of illustration, a process involving the use of an anionic polymer such as a vinylsulfonic acid polymer, an ethylene-maleic anhydride polymer or the like as the electrolyte is known.
Such technologies for the manufacture of microcapsules expected to have a sufficient core-retaining property must provide for sufficient both emulsifying power and wall-forming power, in particular. The emulsifying power is the ability of the system to emulsify a hydrophobic core substance into finely divided droplets and has an important bearing on the particle size and, hence, on the pressure resistance of capsules. The wall-forming power is the ability of the system to marshall the amino resin or the like separating from the aqueous phase of a polycondensation system onto the interface with the core droplet to build a tough resin film in situ and is related to the heat resistance, among others, of microcapsules.
The in-situ polymerization technology has room, however, for improvement in regard to these two essential qualities. Thus, since the conventional process does not provide for a sufficient emulsifying effect, the resulting capsules are comparatively large in diameter and, hence, liable to collapse and fail to retain the core material. Moreover, because the wall-forming power of the process is also inadequate, an amino resin polycondensation reaction system is ready to undergo gelation to interfere with undeterred progress of subsequent polycondensation and, hence, with formation of microcapsules. Even if microcapsules could be formed at any rate, the particle size is large and broad in distribution and the capsules are deficient in resistance to pressure and heat.